An inkjet printer includes one or more ink-filled pens that are mounted to a carriage in the printer body. Normally, the carriage is scanned across the width of the printer as paper or other print media is advanced through the printer. Each ink-filled pen includes a printhead that is driven to expel droplets of ink through an array of nozzles in the printhead toward the paper in the printer. The timing and nominal trajectory of the droplets are controlled to generate the desired text or image output and its associated quality.
These scanning-type printers expel ink while the carriage is reciprocated across the width of the paper. Thus, a swath of ink is printed with each scan and the paper is advanced to a new location between printing swaths. Sometimes, however, the paper is not advanced, or is advanced less than a full print swath, so that another pass may be made by the returning carriage to print over some or all of the last-printed swath. Such multiple passes by scanning-type printers are useful for color printing but can lead to undesirable banding in the output due to changes in the order in which the ink colors reach the paper as a result of the alternating directions of carriage movement.
Throughput, which is normally measured in printed pages per minute, is an important design consideration in connection with printers of all types. The goal is to maximize throughput without deleterious effects on print quality.
One way to increase throughput in inkjet printers is to increase the size of the nozzle array on the printhead of the pen so that the width of the swath of printed ink is correspondingly increased. As printhead swaths increase, however, it becomes increasingly difficult to arrange and move the pens so that the spacing between the paper and nozzle arrays on the printheads remains constant, preferably parallel. This parallelism is important for preventing errors in the placement of ink droplets, especially at the margins of the printheads.
It is also possible to increase printed swath size by making larger monolithic printheads. Increasing printhead sizes beyond certain limits, however, can lead to very poor manufacturing yields. Thus, this approach can lead to more expensive printers than the alternative of combining multiple smaller print swaths together to create a larger, multi-pen print swath.
The width of the printed swath may be increased by mounting two or more of the same color pens in a carriage in a manner such that the swaths of both pens are adjacent to one another to effectively double, triple, etc. the width of the resulting printed swath. This approach, however, in-scanning type printers, increases the complexity of the printer architecture because more components are required for ensuring that the paper is held flat relative to the combined swaths of the pens. Also, it is difficult to efficiently group multiple pens in a scanning type printer without increasing the amount of carriage over-travel or increasing the number of swath-advances required for ensuring that all pens scan the entire width of the paper before the carriage reverses for the next scan. The time required for the over-travel diminishes throughput.
The present invention is directed to a method of combining inkjet pens in a printer so that the swath width is enlarged but while maintaining the printheads and paper parallelism, and without requiring large monolithic printheads having economics that may be relatively poor for a given printed swath width. To this end, the paper is carried on a drum and advanced through the printer. Sets of two pens, each set having the same color of ink, are carried near the drum. The two pens arranged in a carriage such that the swath of one pen is adjacent to the swath of the other pen in a direction that is parallel to the drum axis. As a result, the width of the printed swath for a given color is the sum of the swath widths of the adjacent swaths.
A carriage assembly is provided for carrying the pens in the just mentioned arrangement for combining the swath widths of the individual pens. The components of the carriage assembly are such that two pens of the same color ink are precisely positioned relative to each other, thereby to meet a very close tolerance requirement for arranging pens that print adjacent swaths of the same color.
Multiple sets of relatively small pens are carried near the drum so that a full range of colors can be printed. The present invention ensures that all of the pens' printheads are held sufficiently parallel to the paper to allow excellent print quality. The combination of a drum for advancing the print medium and the arrangement of the pens relative to the drum maintains the desired parallelism while increasing overall swath width for attendant increase in throughput. The drum-based approach also reduces the over-travel problem mentioned above.
Other advantages and features of the present invention will become clear upon study of the following portion of this specification and the drawings .